Three-dimensional macro-chip including optical interconnects

ABSTRACT

A multi-chip module (MCM), which includes a three-dimensional (3D) stack of chips that are coupled using optical interconnects, is described. In this MCM, disposed on a first surface of a middle chip in the 3D stack, there are: a first optical coupler, an optical waveguide, which is coupled to the first optical coupler, and a second optical coupler, which is coupled to the optical waveguide. The first optical coupler redirects an optical signal from the optical waveguide to a first direction (which is not in the plane of the first surface), or from the first direction to the optical waveguide. Moreover, the second optical coupler redirects the optical signal from the optical waveguide to a second direction (which is not in the plane of the first surface), or from the second direction to the optical waveguide. Note that an optical path associated with the second direction passes through an opening in a substrate in the middle chip.

GOVERNMENT LICENSE RIGHTS

The United States Government has a paid-up license in this invention andthe right in limited circumstances to require the patent owner tolicense others on reasonable terms as provided for by the terms ofAgreement No. HR0011-08-9-0001 awarded by the Defense Advanced ResearchProjects Administration.

BACKGROUND

1. Field

The present disclosure relates to a multi-chip module (MCM) thataccommodates semiconductor chips. More specifically, the presentdisclosure relates to an MCM that includes a vertical stack ofsemiconductor chips with inter-chip optical interconnects.

2. Related Art

Engineers have recently proposed using a multi-chip module (MCM) (whichis sometimes referred to as a ‘macro-chip’) to integrate a collection ofsemi-conductor chips. This MCM offers unprecedented computationaldensity, energy efficiency, bisection bandwidth and reduced messagelatencies. These characteristics are obtained by photonicallyinterconnecting multiple silicon chips into a logically contiguous pieceof silicon. This interconnection technique facilitates integration ofcomputer system components, such as: multi-core, multi-threadedprocessors, system-wide interconnects and dense memories.

As shown in FIG. 1, in one configuration of MCM 100, island chips 110and bridge chips 112, are arranged in a two-dimensional, multi-tieredarray. In this MCM, an upward-facing island chip (such as island chip110-1) in the lower tier in MCM 100 is coupled to a downward-facingbridge chip (such as bridge chip 112-1) in the upper tier. Inparticular, in the regions where these chips overlap, communicationoccurs via proximity communication of optical signals (which is referredto as ‘optical proximity communication’ or OPxC).

Because the optical proximity communication occurs between activesurfaces, island chips 110 and bridge chips 112 need to face each other.Consequently, the number of chip layers in MCM 100 is typically limitedto two. However, this limitation constrains potential improvements inthe device density and chip functionality that can be obtained, andthus, the performance, form factor and cost of MCM 100.

Hence, what is needed is an MCM without the above-described limitations.

SUMMARY

One embodiment of the present disclosure provides a multi-chip module(MCM) that includes a first substrate having a first surface and asecond surface. This first substrate includes an opening that extendsthrough the first substrate, where the opening is defined by an edge inthe first surface, an edge in the second surface, and a side wall.Furthermore, disposed on the first surface, the first substrateincludes: a first optical waveguide; a first optical coupler, which isoptically coupled to the first optical waveguide; and a second opticalcoupler, which is optically coupled to the first optical waveguide. Thefirst optical coupler redirects an optical signal from the first opticalwaveguide to a first direction, or from the first direction to the firstoptical waveguide, where the first direction is other than in the planeof the first surface. Moreover, the second optical coupler redirects theoptical signal from the first optical waveguide to a second direction,or from the second direction to the first optical waveguide, where thesecond direction is other than in the plane of the first surface. Notethat an optical path associated with the second direction passes throughthe opening.

In some embodiments, a given optical coupler, which can be either thefirst optical coupler or the second optical coupler, includes a gratingelement. This grating element may have a spatially varying index ofrefraction having a constant fundamental spatial wavelength or a varyingfundamental spatial wavelength. Furthermore, the spatially varying indexof refraction may be associated with a partially etched or a fullyetched layer disposed on the first surface.

Moreover, the first optical coupler may be different than the secondoptical coupler. For example, the second optical coupler may include anoxide layer and/or a metal backend layer. Note that the first opticalcoupler and the second optical coupler may be fabricated using a commonmanufacturing process. This may allow the first optical coupler and thesecond optical coupler to be processed simultaneously, which can reducemanufacturing time and expense.

Furthermore, the first direction may be in a first half of space, andthe second direction may be in a second half of space (such as upperhemisphere and a lower hemisphere, or left hemisphere and righthemisphere).

In some embodiments, the MCM includes a second substrate having a thirdsurface, which faces the first surface. Disposed on the third surface,the second substrate may include: a second optical waveguide; and athird optical coupler, which is optically coupled to the second opticalwaveguide. This third optical coupler may redirect the optical signalfrom the first optical coupler to the second optical waveguide, or fromthe second optical waveguide toward the first optical coupler.

Additionally, the MCM may include a third substrate having a fourthsurface, which faces the second surface. Disposed on the fourth surface,the third substrate may include: a third optical waveguide; and a fourthoptical coupler, which is optically coupled to the third opticalwaveguide. This fourth optical coupler may redirect the optical signalfrom the second optical coupler to the third optical waveguide, or fromthe third optical waveguide toward the second optical coupler.Furthermore, the third substrate may include a layer (other thansilicon) disposed on the fourth surface, and the third optical waveguidemay be defined in the layer. Note that the layer may include a lightsource or active-material regions.

In some embodiments, there are negative features disposed on the firstsurface, the second surface, the third surface and the fourth surface.In addition, the MCM may include positive features that are configuredto mechanically couple the first substrate and the second substrate,and/or the first substrate and the third substrate, by mating with pairsof the negative features. For example, the positive features may includemicro-spheres.

Alternatively, there may be negative features disposed on at least oneof the first surface and the third surface, and at least one of thesecond surface and the fourth surface. Furthermore, there may bepositive features disposed on the other of the first surface and thethird surface, and the other of the second surface and the fourthsurface. These positive features may mechanically couple the firstsubstrate and the second substrate, and/or the second substrate and thethird substrate, by mating with the negative features.

Another embodiment provides a system that includes the MCM.

Another embodiment provides a first method for optically coupling anoptical signal to and from the first substrate. During the first method,the optical signal is redirected from the first direction to the firstoptical waveguide using the first optical coupler, where the firstdirection is other than in the plane of the first surface of thesubstrate, and where the first optical waveguide and the first opticalcoupler are disposed on the first surface. Then, the optical signal istransported in the first optical waveguide to the second opticalcoupler, where the second optical coupler is disposed on the firstsurface. Next, the optical signal is redirected to the second directionusing the second optical coupler, where the second direction is otherthan in the plane of the first surface. Note that the optical pathassociated with the second direction passes through the opening thatextends through the first substrate from the first surface to the secondsurface of the first substrate. Furthermore, the opening is defined bythe edge in the first surface, the edge in the second surface, and theside wall.

Another embodiment provides a second method for optically coupling anoptical signal to and from the first substrate. During the first method,the optical signal is redirected from the second direction to the firstoptical waveguide using the second optical coupler. Then, the opticalsignal is transported in the first optical waveguide to the firstoptical coupler. Next, the optical signal is redirected to the firstdirection using the first optical coupler.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram illustrating an existing multi-chip module(MCM).

FIG. 2 is a block diagram illustrating an MCM in accordance with anembodiment of the present disclosure.

FIG. 3A is a block diagram illustrating an optical coupler for use inthe MCM of FIG. 2 in accordance with an embodiment of the presentdisclosure.

FIG. 3B is a block diagram illustrating an optical coupler for use inthe MCM of FIG. 2 in accordance with an embodiment of the presentdisclosure.

FIG. 4A is a block diagram illustrating an MCM in accordance with anembodiment of the present disclosure.

FIG. 4B is a block diagram illustrating an MCM in accordance with anembodiment of the present disclosure.

FIG. 5 is a block diagram illustrating an MCM in accordance with anembodiment of the present disclosure.

FIG. 6A is a block diagram illustrating an operation in a process forfabricating an MCM in accordance with an embodiment of the presentdisclosure.

FIG. 6B is a block diagram illustrating an operation in the process ofFIG. 6A in accordance with an embodiment of the present disclosure.

FIG. 6C is a block diagram illustrating an operation in the process ofFIG. 6A in accordance with an embodiment of the present disclosure.

FIG. 6D is a block diagram illustrating an operation in the process ofFIG. 6A in accordance with an embodiment of the present disclosure.

FIG. 7 is a flow chart illustrating a process for optically coupling anoptical signal using an MCM in accordance with an embodiment of thepresent disclosure.

FIG. 8 is a flow chart illustrating a process for optically coupling anoptical signal using an MCM in accordance with an embodiment of thepresent disclosure.

FIG. 9 is a block diagram illustrating a system in accordance with anembodiment of the present disclosure.

Note that like reference numerals refer to corresponding partsthroughout the drawings. Moreover, multiple instances of the same partare designated by a common prefix separated from an instance number by adash.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the disclosure, and is provided in the contextof a particular application and its requirements. Various modificationsto the disclosed embodiments will be readily apparent to those skilledin the art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present disclosure. Thus, the present disclosure is notintended to be limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles and features disclosedherein.

Embodiments of a multi-chip module (MCM), a system that includes theMCM, and a technique for coupling an optical signal using the MCM aredescribed. This MCM includes a three-dimensional (3D) stack of chipsthat are coupled using optical interconnects. In particular, disposed ona first surface of a middle chip in the 3D stack there are: a firstoptical coupler; an optical waveguide, which is coupled to the firstoptical coupler; and a second optical coupler, which is coupled to theoptical waveguide. The first optical coupler redirects an optical signalfrom the optical waveguide to a first direction (which is not in theplane of the first surface), or from the first direction to the opticalwaveguide. Moreover, the second optical coupler redirects the opticalsignal from the optical waveguide to a second direction (which is not inthe plane of the first surface), or from the second direction to theoptical waveguide. Note that an optical path associated with the seconddirection passes through an opening in a substrate in the middle chip.

In this way, this optical coupling technique facilitates 3D chipstacking Moreover, by facilitating 3D chip stacking in the MCM, theoptical coupling technique may overcome the two-chip layer constraint inMCM 100 (FIG. 1), thereby increasing the device density and thecapacity-to-volume ratio. Consequently, this optical coupling techniquemay facilitate: smaller form factors (relative to 2D architectures);shorter communication path lengths, which improve inter- and intra-chipcommunication (and, thus, reduce latency and increase performance);reduced cost; and/or reduced power consumption. Furthermore, the MCM mayprovide a platform for hybrid integration of different functional layers(such as transmission, memory, logic, MEMS, exotic substrate material,etc.) and/or components (such as electronic integrated circuits or ICsand/or photonic ICs). These layers and components can be fabricatedusing different processing nodes in a manufacturing process, which maybe independently optimized.

We now describe embodiments of an MCM. FIG. 2 presents a block diagramillustrating an MCM 200. This MCM includes 3D chip-stacking opticalinterconnects based on optical couplers 218 (such as grating elements),optical waveguide 222 and at least one through-substrate opening 228. Inparticular, MCM 200 includes three chips or dies, having substrates 210,which communicate one or more optical signals, such as optical signal226, with each other using ‘optical proximity communication’ or OPxC.

In MCM 200, substrate 210-2 has surfaces 212-2 and 212-3, as well asopening 228, which extends through substrate 210-2 from surface 212-2 tosurface 212-3. This opening is defined by an edge in surface 212-2, anedge in surface 212-3, and side wall 232. Furthermore, disposed onsurface 212-2, substrate 210-2 includes: optical waveguide 222; opticalcoupler 218-2, which is optically coupled to optical waveguide 222; andoptical coupler 218-3, which is optically coupled to optical waveguide222. Optical coupler 218-2 redirects optical signal 226 from opticalwaveguide 222 to or toward a direction 230-1 (which is perpendicular toa plane of surface 212-2, and points away from substrate 210-2), or fromdirection 230-1 to optical waveguide 222 (i.e., in the plane of surface212-2).

Optical signal 226 may be received or provided by optical coupler 218-1,which is disposed on surface 212-1 of substrate 210-1. Note that surface212-1 faces surface 212-2. As with optical coupler 218-2, opticalcoupler 218-1 may redirect optical signal 226 from optical coupler 218-2to an optical waveguide (not shown), which is disposed on surface 212-1.Alternatively, optical coupler 218-1 may redirect optical signal 226from this optical waveguide toward optical coupler 218-2.

Furthermore, optical coupler 218-3 redirects optical signal 226 fromoptical waveguide 222 to or toward direction 230-2 (which isperpendicular to a plane of surface 212-2, and points toward substrate210-2, i.e., which is the opposite direction to direction 230-1), orfrom direction 230-2 to optical waveguide 222. Note that an optical pathassociated with direction 230-2 passes through opening 228 (i.e.,through substrate 210-2).

Optical signal 226 may be received or provided by optical coupler 218-4,which is disposed on surface 212-4 of substrate 210-3. Note that surface212-4 faces surface 212-3. As with optical coupler 218-3, opticalcoupler 218-4 may redirect optical signal 226 from optical coupler 218-3to an optical waveguide (not shown), which is disposed on surface 212-4.Alternatively, optical coupler 218-4 may redirect optical signal 226from this optical waveguide toward optical coupler 218-3.

Chips in MCM 200 may be fabricated using a wide variety of materials andprocessing techniques, as is known to one skilled in the art. In someembodiments, substrates 210 are a semiconductor, such as silicon.(However, in other embodiments, other materials, such as glass orsapphire, are used.) Electrical components may be built up on thesesubstrates using a complementary-metal-oxide-semiconductor (CMOS)process, and optical components may be built up on these substratesusing a silicon-on-insulator (SOI) process. In these embodiments,optical waveguides, such as optical waveguide 222, are in active layers216 (such as active layer 216-2), which may be SO/layers.

Furthermore, oxide layers 214 and backend layers 220 (such as a metaland/or inter-metal dielectric or IMD) may be used to define the opticalcomponents. For example, as described further below with reference toFIGS. 3A and 3B, a spatially varying index of refraction in opticalcouplers 218 may be provided by alternating the active layer and theoxide layer (such as silicon oxide). In addition, opening 228 may befilled with an optically transparent material (over the wavelengthsassociated with optical signal 226), such as an oxide, or may be filledwith a gas, such as air.

Note that optical coupler 218-3 may be different from optical coupler218-2 (and than optical couplers 218-1 and 218-4). In particular,backend layers 220 may be optically transparent (over the wavelengthsassociated with optical signal 226) over optical couplers 218-1, 218-2and 218-4. However, there may be a different, optically opaque coating(over the wavelengths associated with optical signal 226) over opticalcoupler 218-3. For example, as shown in FIG. 2, there may be a backendmetal 224 (such as aluminum or gold) over optical coupler 218-3.Alternatively or additionally, this opaque coating may be implementedusing backend metal 224, oxide and/or IMD, which may reduce the cost andfabrication complexity of MCM 200. Nonetheless, in some embodimentsoptical couplers 218 are fabricated using a common manufacturingprocess, which may reduce the cost of MCM 200.

These differences in optical couplers 218-2 and 218-3 allow thesecomponents to redirect light in different directions 230. In conjunctionwith opening 228, this allows more than two chips to be verticallystacked in MCM 200.

FIG. 3A presents a block diagram illustrating an optical coupler 300 foruse in MCM 200 (FIG. 2). Optical signal 226 carrying information maypropagate from waveguide 222 into optical coupler 300, where analternating pattern of higher and lower index-of-refraction materials(provided by active layer 216-1 and oxide layer 214-1) may redirect itout of the plane of a surface of a substrate on which optical coupler300 is disposed. In particular, when light passes through opticalcoupler 300, the spatially varying index of refraction may scatter lighttowards both the top surface and the substrate (i.e., towards oxidelayer 214-1 underneath the grating element). Moreover, the spatiallyvarying index of refraction may be chosen so that the light is opticallycoupled away from the substrate. For example, the profile or spatialpattern of the grating element (see below) and/or a thickness of oxidelayer 214-1 may be designed so that there is constructive interferencebetween reflected light and the original beam. Alternatively, bydepositing an appropriate metal layer in the backend layer, the lightscattered towards the top surface can be reflected back towards thesubstrate.

As shown in FIG. 3A, in some embodiments an optical coupler (such asoptical coupler 300) may have a periodic structure characterized byspacing 310. In particular, optical coupler 300 may have a spatiallyvarying index of refraction having a constant fundamental spatialwavelength. However, as shown in FIG. 2, in some embodiments opticalcouplers 218 (FIG. 2) have a varying fundamental spatial wavelength(such as a chirp configuration).

Note that the grating element or diffraction grating in embodiments ofthe optical coupler may be defined using a full etch, i.e., an etchdepth 312 that goes all the way through active layer 216-1. However, asshown in FIG. 3B, which presents a block diagram illustrating an opticalcoupler 350 for use in MCM 200 (FIG. 2), a partial etch may be used,i.e., an etch depth 360 that does not go all the way through activelayer 216-1.

While FIGS. 3A and 3B illustrate optical coupling of an optical signalfrom waveguide 222 out of the plane, in other embodiments thepropagation direction is reversed. Furthermore, while FIG. 2, and FIGS.3A and 3B illustrate passive optical couplers, in some embodiments theoptical couplers are actively controlled. For example, control logic mayprovide signals that electronically determine whether or not a givenoptical coupler redirects optical signal 226 out of the plane.

In some embodiments, the MCM includes structures that facilitatealignment of chips in a vertical stack. For example, FIG. 4A presents ablock diagram illustrating an MCM 400 that includes negative features(such as negative features 410) that are defined on surfaces of thechips. These negative features may mechanically couple to positivefeatures (such as positive feature 412-1), thereby mechanically couplingand aligning substrates 210. In particular, the positive features may bemicro-spheres (such as micro-solder balls) that mechanically couple toor mate with pairs of negative features (such as negative features 410-1and 410-2).

Another embodiment is shown in FIG. 4B, which presents a block diagramillustrating an MCM 450. In this MCM, negative features (such asnegative feature 460-1) may be disposed on either or both of the facingsurfaces in a pair of chips. Furthermore, if a negative feature (such asnegative feature 460-1) is disposed on surface 212-1, there may be acorresponding positive feature (such as positive feature 462-1) which isdisposed on facing surface 212-2. These positive and negative featuresmay mechanically couple to or mate with each other, thereby mechanicallycoupling and aligning substrates 210.

In some embodiments, at least one of the substrates 210 includes anadditional material, such as: an exotic material, an epitaxial layerand/or a multiple-layer structure. Thus, in contrast with FIGS. 2, 4Aand 4B, the chips in the MCM may not all include identical materials orlayer structures. This is shown in FIG. 5, which presents a blockdiagram illustrating an MCM 500. In particular, layer 510 is disposed onsurface 212-4 of substrate 210-3. This layer may include a materialother than silicon or an oxide. For example, layer 510 may include:indium phosphide, gallium arsenide or an optically active material. Inthese embodiments, layer 510 may include a light source or lightemitter, such as an on-chip laser.

We now describe embodiments of processes. FIGS. 6A-6D present operationsin a process for fabricating an MCM (such as MCM 400 in FIG. 4) with 3Dchip-stacking optical interconnects. In FIG. 6A, a wafer containingsubstrate 210-1 is fabricated in operation 600. During this operation,one or more backend layer(s) are removed and refilled with a uniformoptically-transparent material, such as silicon oxide, in the vicinityof optical coupler 218-1. Furthermore, alignment components andconnectors (such as positive and negative features) are fabricated orcoupled to substrate 210-1.

Then, in operation 620 in FIG. 6B, another wafer (which is fabricated inits own process) is flipped over to be mechanically coupled to andaligned with substrate 210-1. Note that wafer-level or die-levelflip-chip bonding or pit-ball alignment may be used during thisoperation.

Moreover, in operation 640 in FIG. 6C, a portion of substrate 210-2 (ora carrier) is removed (for example, using chemical mechanical polishing)to expose surface 212-3 for the next 3D stacking chip. Furthermore,opening 228 may be etched and filled with oxide. Note that opening 228may not require precise mechanical alignment (as long as it does notblock the optical path).

Consequently, a mode shape provided by optical coupler 218-3 may not belimited to a single-mode-fiber (SMF) mode.

Next, in operation 660 in FIG. 6D, surface 212-3 of substrate 210-2 maybe patterned and processed to define alignment components and opticalconnectors suitable for the next chip in the vertical stack.Furthermore, operations 600 (FIG. 6A), 620 (FIG. 6B), 640 (FIG. 6C) and660 may be repeated for additional chips in the vertical stack.

FIG. 7 presents a flow chart illustrating a process 700 for opticallycoupling an optical signal using an MCM, such as MCM 200 (FIG. 2).During this first method, the optical signal is redirected from a firstdirection to an optical waveguide using a first optical coupler(operation 710), where the first direction is other than in the plane ofa first surface of a substrate in the MCM, and where the opticalwaveguide and the first optical coupler are disposed on the firstsurface. Then, the optical signal is transported in the opticalwaveguide to a second optical coupler (operation 712), where the secondoptical coupler is disposed on the first surface. Next, the opticalsignal is redirected to a second direction using the second opticalcoupler (operation 714), where the second direction is other than in theplane of the first surface. Note that an optical path associated withthe second direction passes through an opening that extends through thesubstrate from the first surface to a second surface of the substrate.Furthermore, the opening is defined by an edge in the first surface, anedge in the second surface, and a side wall.

FIG. 8 presents a flow chart illustrating a process 800 for opticallycoupling an optical signal using an MCM, such as MCM 200 (FIG. 2).During this first method, the optical signal is redirected from thesecond direction to the first optical waveguide using the second opticalcoupler (operation 810). Then, the optical signal is transported in theoptical waveguide to the first optical coupler (operation 812). Next,the optical signal is redirected to the first direction using the firstoptical coupler (operation 814).

In some embodiments of the process in FIGS. 6A-6D, as well as processes700 (FIG. 7) and 800 (FIG. 8), there are additional or fewer operations.Moreover, the order of the operations may be changed, and/or two or moreoperations may be combined into a single operation.

Embodiments of the MCM may be used in a wide variety of applications.This is shown in FIG. 9, which presents a block diagram illustrating asystem 900 that includes MCM 910. In general, an MCM may include anarray of chip modules (CMs) or single-chip modules (SCMs), and a givenSCM may include at least one substrate, such as a semiconductor die.Furthermore, the substrate may communicate with other substrates, CMs,SCMs, and/or optical devices in the MCM using: optical proximitycommunication, proximity communication of capacitively coupled signals,proximity communication of inductively coupled signals, and/or proximitycommunication of conductively coupled signals.

Furthermore, embodiments of the MCM may be used in a variety ofapplications, including: VLSI circuits, communication systems (such asin wavelength division multiplexing), storage area networks, datacenters, networks (such as local area networks), and/or computer systems(such as multiple processor-core computer systems). For example, an MCMmay be included in a backplane that is coupled to multiple processorblades, or an MCM may couple different types of components (such asprocessors, memory, input/output devices, and/or peripheral devices). Insome embodiments, an MCM performs the functions of: a switch, a hub, abridge, and/or a router.

Note that system 900 may include, but is not limited to: a server, alaptop computer, a communication device or system, a personal computer,a work station, a mainframe computer, a blade, an enterprise computer, adata center, a portable-computing device, a supercomputer, anetwork-attached-storage (NAS) system, a storage-area-network (SAN)system, and/or another electronic computing device. Moreover, note thata given computer system may be at one location or may be distributedover multiple, geographically dispersed locations.

MCM 200 (FIG. 2), MCM 400 (FIG. 4A), MCM 450 (FIG. 4B), MCM 500 (FIG. 5)and/or system 900 may include fewer components or additional components.For example, in some embodiments, instead of fabricating athrough-substrate optical via (opening 228 in FIG. 2), substrate 210-2(FIGS. 2, 4A, 4B and 5) may be removed completely, such as by using alift-off process. Furthermore, more than one substrate 210 (FIG. 2) mayinclude an opening (in particular, in embodiments where there are morethan three chips). Alternatively or additionally, there may beadditional layers between optical waveguides (such as waveguide 222 inFIG. 2) and substrates 210 (FIGS. 2, 4A, 4B and 5). Thus, a layer orcomponent ‘disposed on a surface’ should be understood to include thelayer or the component being directly deposited on the surface, or beingdeposited on one or more intermediate layers that are between the layeror the component and the surface. Note that while FIGS. 2 and 4Aillustrate chips having a similar number of layers and layerthicknesses, in some embodiments, depending on the specific functions ofthese chips and the manufacturing processes used to fabricate them, thismay not be the case.

Furthermore, although these MCMs and system 900 are illustrated ashaving a number of discrete items, they are intended to be functionaldescriptions of the various features that may be present rather thanstructural schematics of the embodiments described herein. Consequently,in these embodiments two or more components may be combined into asingle component, and/or a position of one or more components may bechanged.

In some embodiments, the active layers 216 (FIG. 2) on substrates 210(FIGS. 2, 4A, 4B and 5) include additional components, such as: anoptical source (such as a laser), a modulator, a router, a multiplexer(such as an add filter), a de-multiplexer (such as a drop filter), adetector, an amplifier, a filter, and/or a switch. For example, theremay be an add filter for use in wavelength-division multiplexing.Moreover, the additional components may be associated with: a network, abus and/or a point-to-point link. These components may be implementedusing optical components and/or electrical circuits.

The foregoing descriptions of embodiments of the present disclosure havebeen presented for purposes of illustration and description only. Theyare not intended to be exhaustive or to limit the present disclosure tothe forms disclosed. Accordingly, many modifications and variations willbe apparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present disclosure. The scope ofthe present disclosure is defined by the appended claims.

1. A multi-chip module (MCM), comprising a first substrate having afirst surface and a second surface, wherein the first substrate includesan opening that extends through the first substrate, wherein the openingis defined by an edge in the first surface, an edge in the secondsurface, and a side wall, and wherein, disposed on the first surface,the first substrate includes: a first optical waveguide; a first opticalcoupler, optically coupled to the first optical waveguide, configured toredirect an optical signal from the first optical waveguide to a firstdirection, or from the first direction to the first optical waveguide,wherein the first direction is other than in the plane of the firstsurface; and a second optical coupler, optically coupled to the firstoptical waveguide, configured to redirect the optical signal from thefirst optical waveguide to a second direction, or from the seconddirection to the first optical waveguide, wherein the second directionis other than in the plane of the first surface; and wherein an opticalpath associated with the second direction passes through the opening. 2.The MCM of claim 1, wherein a given optical coupler, which can be eitherthe first optical coupler or the second optical coupler, includes agrating element.
 3. The MCM of claim 2, wherein the grating elementincludes a spatially varying index of refraction having a constantfundamental spatial wavelength.
 4. The MCM of claim 2, wherein thegrating element includes a spatially varying index of refraction havinga varying fundamental spatial wavelength.
 5. The MCM of claim 2, whereinthe grating element includes a spatially varying index of refractionwhich is associated with a fully etched layer disposed on the firstsurface.
 6. The MCM of claim 2, wherein the grating element includes aspatially varying index of refraction which is associated with apartially etched layer disposed on the first surface.
 7. The MCM ofclaim 1, wherein the first optical coupler is different than the secondoptical coupler.
 8. The MCM of claim 7, wherein the first opticalcoupler and the second optical coupler are fabricated using a commonmanufacturing process.
 9. The MCM of claim 1, wherein the second opticalcoupler includes an oxide layer or a metal backend layer.
 10. The MCM ofclaim 1, wherein the first direction is in a first half of space and thesecond direction is in a second half of space.
 11. The MCM of claim 1,further comprising a second substrate having a third surface, whichfaces the first surface, wherein, disposed on the third surface, thesecond substrate includes: a second optical waveguide; and a thirdoptical coupler, optically coupled to the second optical waveguide,configured to redirect the optical signal from the first optical couplerto the second optical waveguide, or from the second optical waveguidetowards the first optical coupler.
 12. The MCM of claim 11, whereinthere are negative features disposed on the first surface and the thirdsurface; and wherein the MCM further includes positive features that areconfigured to mechanically couple the first substrate and the secondsubstrate by mating with pairs of the negative features.
 13. The MCM ofclaim 12, wherein the positive features include micro-spheres.
 14. TheMCM of claim 11, wherein there are negative features disposed on one ofthe first surface and the third surface; wherein there are positivefeatures disposed on the other of the first surface and the thirdsurface; and wherein the positive features are configured tomechanically couple the first substrate and the second substrate bymating with the negative features.
 15. The MCM of claim 11, furthercomprising a third substrate having a fourth surface, which faces thesecond surface, wherein, disposed on the fourth surface, the thirdsubstrate includes: a third optical waveguide; and a fourth opticalcoupler, optically coupled to the third optical waveguide, configured toredirect the optical signal from the second optical coupler to the thirdoptical waveguide, or from the third optical waveguide toward the secondoptical coupler.
 16. The MCM of claim 15, wherein the third substrateincludes a layer, other than silicon, disposed on the fourth surface;and wherein the third optical waveguide is defined in the layer.
 17. TheMCM of claim 16, wherein the layer includes a light source.
 18. The MCMof claim 15, wherein there are negative features disposed on the secondsurface and the fourth surface; and wherein the MCM further includespositive features that are configured to mechanically couple the firstsubstrate and the third substrate by mating with the negative features.19. A system, comprising a MCM, wherein the MCM includes a firstsubstrate having a first surface and a second surface, wherein the firstsubstrate includes an opening that extends through the first substrate,wherein the opening is defined by an edge in the first surface, an edgein the second surface, and a side wall, and wherein, disposed on thefirst surface, the first substrate includes: a first optical waveguide;a first optical coupler, optically coupled to the first opticalwaveguide, configured to redirect an optical signal from the firstoptical waveguide to a first direction, or from the first direction tothe first optical waveguide, wherein the first direction is other thanin the plane of the first surface; and a second optical coupler,optically coupled to the first optical waveguide, configured to redirectthe optical signal from the first optical waveguide to a seconddirection, or from the second direction to the first optical waveguide,wherein the second direction is other than in the plane of the firstsurface; and wherein an optical path associated with the seconddirection passes through the opening.
 20. A method for opticallycoupling an optical signal to and from a substrate, comprising:redirecting an optical signal from a first direction to an opticalwaveguide using a first optical coupler, wherein the first direction isother than in a plane of a first surface of the substrate, and whereinthe optical waveguide and the first optical coupler are disposed on thefirst surface; transporting the optical signal in the optical waveguideto a second optical coupler, wherein the second optical coupler isdisposed on the first surface; and redirecting the optical signal to asecond direction using the second optical coupler, wherein the seconddirection is other than in the plane of the first surface; wherein anoptical path associated with the second direction passes through anopening that extends through the substrate from the first surface to asecond surface of the substrate; and wherein the opening is defined byan edge in the first surface, an edge in the second surface, and a sidewall.